The Influence of Basicranial Shape on Facial Asymmetry

Objectives: Facial asymmetry is a common clinical problem with a heterogeneous etiological basis. The basicranium is a developmentally distinct component of the skull that ossifies early in development and is thought to act as a platform for later facial growth. We hypothesized that basicranial shape influences facial shape and asymmetry through either direct additive effects or indirect compensatory growth in the facial skeleton. To test these alternative hypotheses, we evaluated covariation of facial and basicranial asymmetry in clinical populations of humans and in mice in which chondrocranial development had been modified through the cre-conditional deletion of Pten in Col2a1 expressing cells.
Methods: We collected 3D landmark data for the basicranium and facial skeleton of pre-treatment orthodontics patients from UCSF (n=87) and mice with mutations in chondrocranial development. We next performed Procrustes Superimposition to remove the effects of scale and orientation, and performed a Partial Least Squares (PLS) analysis on the Procrustes data to test for significant covariation in the asymmetric variation of the basicranium and face. Covariation was visualized by applying associated shape vectors to the mean shape.
Results: Asymmetric variation between the basicranium and face exhibited significant covariation in both humans (RV=0.452, p<0.0001) and mouse (RV=0.365, p=0.004). In both cases, the first PLS axis explained a significant proportion of total covariation (human: 59.7%, r=0.902, p<0.001; mouse: 51.9%, r=0.740, p=0.036). In both populations there was an additive relationship in which the side with the larger basicranium also had a larger face.
Conclusions: Our results support the hypothesis that basicranial shape influences facial asymmetry. In both the human and mouse population, there appears to be an additive effect in which facial asymmetry is in the same direction but higher magnitude.